Conventional digital images for diagnostic medical imaging systems traditionally use on the order of 256 gray scale display levels to produce the image on a cathode ray tube (CRT). These 256 display levels are often allocated to more, or fewer, information levels of the original digital data by windowing the information levels through a digital look-up table that contains some desired data level-to-display level correspondence function, i.e. a gray scale transfer curve. The usual purpose of such windowing is to increase selected contrast ratios in the displayed image over what they would be without windowing. For example, in an x-ray image containing 256 digital information levels, the information levels for normal tissue in a particular organ might cluster around level 200, while diseased tissue in that organ would tend to have information levels clustered around level 210. The contrast ratio between the diseased and normal tissue is therefore (210-200)/200 or 5%--a value that may or may not be easily perceived by an observer, depending on various factors such as the sharpness of the boundary between the diseased and normal tissue, displayed image brightness, glare from other parts of the image etc.
If the display system gray scale window is set so that only data levels from 150 to 250 are assigned with a linear relationship to the 256 display levels, then the normal tissue information level of 200 would be displayed at level 127, while the diseased tissue level of 210 would now be displayed at level 153, giving a contrast ratio of 20%, or 4 times as much as before.
The problem with this windowing technique is that while contrast ratios for information levels within the window range are enhanced, information levels outside the window range cannot be viewed simultaneously, forcing the operator to sequentially examine successive information level ranges of the same image, and mentally integrate all of them into a diagnosis. This is, at best, a time-consuming and difficult procedure requiring considerable skill, as a high-quality x-ray CRT image can have up to 2,048 information levels to cover the density range from bone to air. The conventional windowing technique therefore presents a trade-off between a high-contrast display of a narrow dynamic range of information levels, and a low-contrast display of a wide dynamic range of information levels.
Another problem with conventional windowing techniques is that under some conditions, high contrast ratios for single steps of the lower display levels can be perceived by the eye, as an annoying contour effect along sharp edges. To avoid such contouring, it is desirable to keep the single-step contrast ratio below about 2% in even the darkest portions of the image.
A third problem occurs with many video display systems when it is desired to photograph the image using negative transparency film. To produce the same polarity film image as was viewed on the monitor, it is necessary to invert the relationship between the image information levels and the displayed gray scale levels, using a non-linear transfer curve. The small contrast ratios contained in the upper information levels selected for windowing are often poorly reproduced in the inverted gray scale display because of insufficient resolution in the lower, or dark, portion of the displayed levels. Depending upon the inversion transfer function used, significant loss of image information may occur in the photographic copy because of the round-off error.